Tantalum is a highly corrosion resistant, bio-friendly metal. As a result it finds wide use in reactors, heat exchangers, piping and the like in the chemical and pharmaceutical processing industries. Because tantalum is very expensive often the structural components used in this equipment are made up of a steel or stainless steel section for strength purposes that is clad with a thin sheet of tantalum to prevent interaction with the process fluid. In order for the tantalum sheet to provide corrosion protection for a whole vessel many such sheets must be joined together into a single impermeable piece. Many techniques have been used but all of them are costly, have severe deficiencies, and cause the cost of the basic tantalum clad steel section to be higher than necessary. This invention provides a low cost, chemical resistant, material and manpower efficient means of joining the tantalum sheets together.
Cold spray or kinetic spray (see U.S. Pat. Nos. 5,302,414, 6,502,767 and 6,759,085) is an emerging industrial technology that is being employed to solve many industrial manufacturing challenges (see, e.g., U.S. Pat. Nos. 6,924,974, 6,444,259, 6,491,208 and 6,905,728). Cold spray employs a high velocity gas jet to rapidly accelerate powder particles to high velocity such that when they impact a surface the particles bond to the surface to form an integral, well bonded and dense coating. The cold spraying of tantalum powders onto a variety of substrates (including steel) has been suggested (see, e.g., “Analysis of Tantalum Coatings Produced by the Kinetic Spray Process,” Van Steenkiste et al, Journal of Thermal Spray Technology, volume 13, number 2, June 2004, pages 265-273; “Cold spraying—innovative layers for new applications,” Marx et al, Journal of Thermal Spray Technology, volume 15, number 2, June 2006, pages 177-183; and “The Cold Spray Process and Its Potential for Industrial Applications,” Gärtner et al, Journal of Thermal Spray Technology, volume 15, number 2, June 2006, pages 223-232). This is all accomplished without having to heat the tantalum powder to a temperature near or above its melting point as is done with traditional thermal spray processes. The fact that dense coatings can be formed at low temperatures present many advantages. Such advantages include reduced oxidation, high density deposits, solid state compaction, the lack of thermally induced stresses and particularly, in this case, the lack of substrate heating. This is critical because at elevated temperatures, such as in a molten Ta weld pool, Ta can dissolve the elemental components of steels and stainless steels with the result that brittle and non corrosion resistant phases form in the tantalum.
As mentioned above, tantalum is a preferred corrosion resistant material in industries that process chemically aggressive liquids. Because of tantalum's high cost rather than being used as a thick structural member, it is frequently used in thin layers as a protective cladding on steel or stainless steel. Many techniques have been developed to attach tantalum clad to the substrate material such as high temperature brazing (U.S. Pat. No. 4,291,104), low temperature soldering (U.S. Pat. No. 4,011,981), diffusion bonding (U.S. Pat. No. 5,693,203), explosive bonding (U.S. Pat. No. 4,291,104), and flouroelastomers (U.S. Pat. No. 4,140,172).
The fabrication problem becomes difficult and expensive however when the individually clad components must be joined together to form a fully functional vessel such as a process reactor. The clad must be fused together to prevent the process liquid from contacting the steel, the steel must also be fused to provide strength to the entire structure. Dissolution of the steel into the tantalum during either fusion process would immediately destroy the desirable properties of tantalum. Thus, all high temperature fusion processes require and use elaborate joint designs and fabrication techniques in order to prevent the tantalum from reaction with the steel during the fusion process.
U.S. Pat. No. 4,073,427 solves the problem of joining the sheets by providing a machined groove around the entire perimeter of all of the structural parts to be joined. The structural steel is then welded and a machined tantalum batten is inserted in the groove to isolate the tantalum from the steel when the tantalum is welded. The tantalum sheet can then be bent flat (it has to be bent up initially to allow insertion of the batten) and the final tantalum weld performed. Further due to the high temperatures involved purge holes must be provided in the steel element to allow for the introduction of inert gases to protect the final tantalum weld.
U.S. Pat. No. 4,818,629 provides a similar approach except that three battens are used, one steel and two tantalum battens. This approach requires two welds as well as multiple purge holes drilled in the steel backing sheet.
U.S. Pat. No. 5,305,946 attempts to improve on the above processes by providing a wide single batten that completely fills the gap between the two tantalum sheets and then double welding a tantalum closure across the top of the tantalum sheets. All of the references discussed so far are done so in terms of joining flat sections. The problems become far more difficult when welding rings to rings to form long vessels, or domes to rings to provide a pressure closure or even vessel penetrations for piping. Bending the tantalum sheet in a circular pattern, then bending 90 degrees out of the plane of the circle, inserting the batten and bending the two tantalum sheets flat again is time consuming and difficult.
U.S. Pat. No. 4,459,062 describes a process using a plasma arc spray overlay to cover the contaminated weld of the tantalum protective layer. First the steel is welded to itself and then the tantalum is welded to itself. Because the tantalum is in intimate contact with the steel, the steel contaminates the tantalum weld and the corrosion resistance of the weld is greatly reduced. A high temperature plasma arc spray is used to provide a protective layer of tantalum over the contaminated weld. Because these joints are used on large structures they usually must be made in air, thus the hot plasma arc causes both the tantalum sheet and the tantalum powder to oxidize due to the plasma's very high temperature. The result is a porous, lamellar structure in the deposit that is high in oxygen content. The porosity and porous grain boundaries greatly reduce the corrosion resistance by percolation effects and the high oxygen content results in a less ductile (than the tantalum sheet) deposit that is prone to cracking and can fail during operation. Additionally, because the coating is put down hot, and because the coefficient of thermal expansion for tantalum is almost double that of steel, as the structure cools the brittle plasma spray deposit is put in a state of tensile stress, a stress that potentially can lead to cracking and failure of the coating.
U.S. Pat. No. 6,749,002 describes a method of spray joining articles. The patent indicates that cold spray as well as the many types of hot spray forming techniques (such as plasma and twin wire arc spraying) could be used. However, the method is severely limited in that at least one of the articles must be a spray formed steel article and the sprayed particles must be steel particles. Clearly this method is used for making structural steel joints involving at least one steel component. In the invention described below the structural steel joint is provided by traditional welding techniques. In fact there is no desire or requirement that the sprayed material join to the steel. In the present invention, the cold sprayed material, in this case tantalum, is used to join the tantalum surface coating and to provide an impermeable corrosion resistant layer of tantalum between two or more co-existing Ta sheets.
U.S. Pat. No. 6,258,402 is even more limited in that it describes a method for repairing spray formed steel tooling. It too requires a steel spray formed part on which a steel spray formed coating will be used to fill a region which has been damaged or somehow eroded away. Additionally, once the spray forming is complete, the spray formed filler is melted and fused using conventional electric welding processes. The purpose of the invention below is to avoid heating and especially melting of the tantalum during the joining processes.